1. Field of the Invention
The present invention relates to Compton light sources, and more specifically, it relates to pulse formats and interaction geometries that produce ultra narrow bandwidth (10E-3 or lower) and high beam flux quasi-mono-energetic x-rays and gamma rays.
2. Description of Related Art
Gamma-rays and x-rays can be produced via laser Compton scattering in which an energetic laser pulse collides with a relativistic bunch of electrons which have been produced by a particle accelerator. The output of this interaction is up-shifted light that is directed in the direction of the electron beam. The energy of the up-shifted light in a head-on collision is equal to the incident laser photon energy multiplied by 4 times the normalized energy of the electron squared. Up-shifts of a million can be created by electrons with energy of a few hundred MeV. The scattered light is polarized and tunable by changing either the color of the laser photon or the energy of the electron bunch. The output is polychromatic but with a spectrum that is angle correlated. By passing the beam through a narrow aperture a quasi-mono-energetic beam can be created with a bandwidth that is dependent linearly upon the laser bandwidth, linearly upon the electron bunch energy spread and upon the focusing geometry of the electron beam and the laser beam.
Laser-Compton light sources have been constructed primarily to create short duration x-rays or tunable, relatively broadband x-rays. In these systems, the laser pulse duration is of order or shorter in duration than that of the electron bunch and both are focused to a small spot in order to maximize the interaction and the total photon yield. The Compton scattering cross section (also known as the Thomson cross section) is very small, ˜6×1025 cm2. Note in Compton scattering, of the order of 1019 laser photons interact with the order of 1010 electrons to produce of the order of 1010 up-scattered x-rays or gamma-rays. To first order, no laser photons are used. Because of the tight focus, the longitudinal transit time of the electrons through the focal region is typically of order the duration of the electron hunch. In this scenario the laser pulse and electron bunch timing must be carefully adjusted so that both pulses overlap at a common focus in space. Furthermore both the laser pulse energy and the electron beam charge are made as high as practical to increase the probability of interaction. and the flux of the outgoing beam. This process can also be used to make gamma-rays simply by increasing the energy of the electron beam. The generation of gamma-rays can be more efficient in that higher energy electron beams can be focused to smaller spots, thus producing more up-scattered photons. Because of the large laser bandwidth used, the relatively large energy spread of the high charge electron bunches and the tight focusing geometries employed in these systems, the fractional bandwidth of typical laser Compton light sources has been of order 10%. (Measurements from systems at Duke University, the Japanese Atomic Research Agency in Japan and at Lawrence Livermore National Laboratory (LLNL) are in this range).
However for many gamma-ray applications the primary beam quality of interest is not beam pulse duration or even total beam flux but is instead gamma-ray bandwidth. It is desirable to provide gamma-rays with fractional bandwidths of 10E-3 or less for use to uniquely excite narrow band (10E-6) nuclear resonances that are unique signatures of isotopes. By monitoring the absorption of resonance photons from such a laser-Compton gamma-ray beam, one can detect, assay or image the presence of specific isotopes in complex systems. Applications include homeland security, nuclear fuel management, industrial materials processing and medical therapy and radiography.